1. Field of the Invention
The present invention relates to a voice transmission method and apparatus in a duplex (or simultaneous transmission and reception) radio system using one carrier frequency.
2. Description of the Related Art
An example of the construction of the conventional voice transmission apparatus in a single-frequency duplex radio system is shown in FIG. 1. Referring to FIG. 1, a voice signal to be transmitted or a transmit voice signal inputted to a transmit voice input terminal 6 is supplied through a time compression circuit 7 and a splatter filter 4 to a modulating circuit of a transmitter 14 as a signal which modulates a carrier frequency wave in the modulating circuit of the transmitter. On the other hand, a demodulated signal of a receiver 15 is outputted as a receive voice signal to a terminal 12 through a time expansion circuit 13. Also, a part of the demodulated output of the receiver 15 is inputted to a timing control circuit 16 through a sync extraction circuit 17. The transmitter 14, the receiver 15, the time compression circuit 7 and the time expansion circuit 13 are controlled by control signals from the timing control circuit 16.
The operation of the radio system shown in FIG. 1 will now be explained briefly by use of FIG. 2. A voice signal to be transmitted, as shown by (a) in FIG. 2, is subjected to 1/N (for example, 1/2) time compression at every constant period of time T to obtain a time-compressed voice signal as shown by (b) in FIG. 2. As a result, a block for compression and a block for pause are generated in the constant time T. The compression block and the pause block are respectively allotted to a transmitting time and a receiving time so as to effect the transmission and reception of a compressed voice signal, as shown by (c) in FIG. 2. In FIG. 2, (d) shows a compressed voice signal received. This signal is time-expanded to two times so that it is restored to a continuous voice signal as shown by (e) in FIG. 2.
The timing for change-over between the time compression and expansion and the timing for change-over between the transmitting and receiving operations are controlled by the timing control circuit 16 using a synchronizing signal. The synchronizing signal is transmitted by a transmitting or calling station and is received by a receiving or called station which in turn extracts the synchronizing signal by means of the sync extraction circuit 17 to make an operation timed to the calling station. The above operation enables a duplex communication (or simultaneous transmission and reception) using a single frequency.
The above-mentioned voice transmission apparatus in the radio system is disclosed by, for example, WO 91/02414 laid open on Feb. 21, 1991.